mx = 0
s = input()
a = [s[0], s[1], s[2], s[3], s[0:2], s[1:3], s[2:4], s[0:3], s[1:4], s]
for j in range(len(a)):
    n = int(a[j])
    c = 0
    for i in range (1, n+1):
        if n%i == 0:
            c = c + 1
    if c == 2:
        mx = max(mx, n)
print(mx)
